Initial Big Bang integration for new istio sandbox chart deploy

After initial deploy/exploration in #2213 (closed), create a long-lived bigbang branch and make appropriate updates to deploy new istio charts via bigbang/umbrella. Other related issues should branch from and merge back into this long-lived branch.

New charts include base, istiod, and gateway.

High level updates would include (at minimum):

• Update chart/values.yaml with new charts
• Add new chart/templates/<package> directories for each new chart and create the following files in each (and any others) as needed. Reference existing packages for examples.
  • git-credentials.yaml
  • gitrepository.yaml
  • helmrelease.yaml
  • imagepullsecret.yaml
  • namespace.yaml
  • values.yaml
• Update chart/values.schema.json to account for new packages

Templating for gateway resources will likely be special to account for multiple instances of the chart being dynamically deployed. Those will be worked separately in #2215 (closed).